Ball screw device

ABSTRACT

A ball screw nut, into which a ball screw shaft is screwed, is coupled with a first cylindrical member disposed in a periphery of the ball screw shaft so as not to rotate with each other. The ball screw nut and the first cylindrical member are rotatably supported on a second cylindrical member by way of balls arranged so as to form two rows. One end portion of the ball screw nut is spline-fitted with one end portion of the first cylindrical member. Two rows of ball bearing raceways are formed on an outer peripheral face of the ball screw nut.

TECHNICAL FIELD

This invention relates to a ball screw device which is used in anelectromagnetic damper or the like of a vehicle.

BACKGROUND ART

It is well-known an electromagnetic damper using a ball screw devicewhich is disclosed in Japanese Patent Publication No. 2005-264992A.

In the ball screw device in this electromagnetic damper, a ball screwnut into which a ball screw shaft is screwed is coupled to a tubularrotary body which is disposed around the ball screw shaft so as not torotate with each other, and the ball screw nut is rotatably supported bya tubular supporting body by a ball bearing. The tubular supporting bodyis provided with a stator of a motor for applying a damping force, andthe tubular rotary body serves as a rotor of this motor.

Moreover, as the ball screw device which is used in this electromagneticdamper, it is well-known a device as shown in FIG. 2.

In FIG. 2, the ball screw device comprises: a ball screw shaft 1; a ballscrew nut 2 into which the ball screw shaft 1 is screwed; a cylindricalrotor forming member 3 coupled to the ball screw nut 2 so as not torotate with each other; a cylindrical housing 5 and a cylindrical statorforming member 6 both of which are fixed to a vehicle body 4; and twoangular ball bearings 7, 8 for rotatably supporting the rotor formingmember 3 with respect to the housing 5.

A lower part of the housing 5 is fitted into a circular hole 9 formed inthe vehicle body 4. An outer periphery of an upper part of the housing 5is formed with a flange 5 a extending outward and fixed to the vehiclebody with a bolt 10. A lower part of the stator forming member 6 isformed with a flange 6 a extending outward and fixed to an upper endface of the flange 5 a of the housing 5 with the bolt 10.

An outer periphery of a lower part of the ball screw nut 2 is providedwith a flange 2 a. A lower end part of the rotor forming member 3 isfitted to a part of the outer periphery of the ball screw nut 2 abovethe flange 2 a, so as not to rotate with each other. A lower end face ofthe rotor forming member 3 is in contact with an upper end face of theflange 2 a. Back faces of the ball bearings 7, 8 are faced each other.Inner rings 7 a, 8 a of these ball bearings 7, 8 are fitted to an outerperiphery of a lower part of the rotor forming member 3 and fixed to therotor forming member 3 by an inner ring fixing nut 11 which is screwedon a male thread part formed in the lower end part of the rotor formingmember 3. A coupling nut 12 is fitted to the outer periphery of thelower end part of the ball screw nut 2, and a male thread part formed onan outer periphery of the coupling nut 12 is screwed on a female threadpart formed on an inner periphery of the ball screw nut 2, whereby theball screw nut 2 and the rotor forming member 3 are coupled to eachother. The outer rings 7 b, 8 b of the bearings 7, 8 are fitted to theinner periphery of the lower part of the housing 5 and fixed to thehousing 5 by an outer ring fixing nut 13 which is screwed on a femalethread part formed on the inner periphery of the lower end part of thehousing 5.

Although not shown in the drawing, a stator of the motor is formed inthe stator forming member 6, and a rotor of the motor is formed in therotor forming member 3. Moreover, the lower end part of the ball screwshaft 1 is coupled to a wheel axle side of the vehicle.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is required that the electromagnetic damper of the vehicle is madecompact and lightweight, for the purpose of decreasing an inertial forceof a rotary part including the ball screw nut.

However, in the above described ball screw device, several nuts arenecessary for coupling the ball screw nut to the rotor forming member,and for supporting the inner and outer rings of the bearing. Therefore,the number of components in the ball screw device is increased, and itis difficult to make the device compact and lightweight. Moreover, thereis such possibility that the nuts are loosened due to high rotationspeed of the ball screw nut. Therefore, since measures such as caulkingthe nuts are required to avoid the above situation, assembling steps areincreased.

An object of this invention is to provide a ball screw device in whichthe above described problems are solved, by reducing the number ofcomponents, whereby it is possible to make the device compact andlightweight, and to decrease assembling steps.

How to Solve the Problems

According to the present invention, there is provided a ball screwdevice, comprising:

a ball screw shaft;

a ball screw nut into which the ball screw shaft is screwed, the ballscrew nut being formed with a plurality of raceways on an outerperipheral face thereof;

a first cylindrical member disposed in a periphery of the ball screwshaft, and having an end portion spline-fitted with one end portion ofthe ball screw nut so as not to rotate with each other;

balls arranged to form a plurality of rows and being rotatable on theraceways of the ball screw nut; and

a second cylindrical member rotatably supporting the ball screw nut andthe first cylindrical member by way of the balls.

Because the ball screw nut is spline-fitted with the first cylindricalmember, a coupling nut required in the conventional configuration can beomitted. Moreover, the double-row raceways of the ball bearing areformed on the outer peripheral face of the ball screw nut, and the innerrings of the ball bearing are integrally formed with the ball screw nut.Accordingly, the inner rings and an inner ring fixing nut required inthe conventional configuration can be omitted.

Thus, the number of components is reduced, so that the ball screw nutcan be made compact and lightweight, and an inertial force of the rotarypart is decreased. Moreover, because the coupling nut and the inner ringfixing nut are not required, caulking of the nuts have becomeunnecessary, and the assembling steps are decreased.

In this ball screw device, for example, the first cylindrical member isformed with a rotor of a motor, and the second cylindrical member isformed with a stator of the motor.

With this configuration, it is available a ball screw deviceincorporated in an electromagnetic damper.

For example, a rotor forming member serves as the first cylindricalmember.

For example, a housing fixed on a stationary part of a vehicle body andsupporting an outer ring of a ball bearing, and a stator forming memberfixed on the housing serve as the second cylindrical member.

ADVANTAGES OF THE INVENTION

According to the ball screw device of the present invention, by reducingthe number of components as describe above, it is possible to make thedevice compact and lightweight, and to decrease the assembling steps,and it is also possible to decrease the inertial force of the rotarypart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section view showing one side of a center lineof a main part of a ball screw device according to one embodiment of theinvention is incorporated in an electromagnetic damper of a vehicle.

FIG. 2 is a longitudinal section view showing one side of a center lineof a main part of a ball screw device incorporated in a conventionalelectromagnetic damper.

DESCRIPTION OF REFERENCE NUMERALS

-   5 housing (second cylindrical member)-   6 stator forming member (second cylindrical member)-   20 ball screw shaft-   21 ball screw nut-   22 rotor forming member (first cylindrical member)-   25 double-row ball bearing-   26, 27 spline teeth-   29 ball-   31 raceway

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, there will be described one embodiment of theinvention applied to an electromagnetic damper of a vehicle.

FIG. 1 is a vertical section view of a main part of a ball screw device,in which only one side of a center line is shown, in the same manner asin FIG. 2. In FIG. 1, components identical with those in theconventional case in FIG. 2 are denoted with the same referencenumerals.

The ball screw device comprises: a ball screw shaft 20; a ball screw nut21 into which the ball screw shaft 20 is screwed; a cylindrical rotorforming member 22 coupled to the ball screw nut 21 so as not to rotatewith each other; a cylindrical housing 5 and a cylindrical statorforming member 6 both of which are fixed to a vehicle body 4; and adouble-row ball bearing 25 for rotatably supporting the ball screw nut21 with respect to the housing 5. The rotor forming member 22 serves asa first cylindrical member. The housing 5 and the stator forming member6 serve as a second cylindrical member.

The housing 5 and the stator forming member 6 are fixed to the vehiclebody 4 by a bolt 10, in the same manner as in the conventional caseshown in FIG. 2.

Spline teeth 26, 27 to be meshed with each other are respectively formedon an outer peripheral face of an upper end part of the ball screw nut21 and on an inner peripheral face of a lower end part of the rotorforming member 22. By meshing these teeth 26, 27 with each other, theball screw nut 21 and the rotor forming member 22 are spline-fitted witheach other.

The bearing 25 is composed of an outer ring 28 and balls 29 arranged toform two rows. An inner ring of the bearing 25 is integrally formed withthe ball screw nut 21. The outer ring 28 of the bearing 25 is fitted toan inner periphery of a lower part of the housing 5, and fixed to thehousing 5 by an outer ring fixing nut 30 which is screwed on a femalethread part formed on the inner periphery of the lower part of thehousing 5. Two rows of raceways 31, 32 are respectively formed on theouter peripheral face of the ball screw nut 21 and an inner peripheralface of the outer ring 28. The balls 29 are interposed between theassociated raceways 31, 32.

Although not shown in the drawing, the stator of the motor is formed inthe stator forming member 6, and the rotor of the motor is formed in therotor forming member 22, in the same manner as in the conventional caseshown in FIG. 2. Moreover, a lower end part of the ball screw shaft 20is coupled to the wheel axle side of the vehicle.

In the ball screw device in the electromagnetic damper as describedabove, vertical movement of the ball screw shaft 20 is converted torotation of the ball screw nut 21, thereby to rotate the rotor formingmember 22 and the rotor formed therein, together with the ball screw nut21. The motor applies a damping force to this rotation to damp thevertical movement of the ball screw shaft 20.

Although, in the above described embodiment, the ball screw nut 21 isrotated by the movement of the ball screw shaft 20, it is also possibleto configure to move the ball screw shaft 20 in a vertical direction byrotating the ball screw nut 21 with the motor.

This invention can be also applied to other devices than theelectromagnetic damper of the vehicle. Moreover, the ball screw deviceaccording to this invention can be applied to both the case where theball screw shaft is rotated and the case where the ball screw nut isrotated.

Structure of the ball screw device is not limited to the above describedembodiment, but can be appropriately modified.

1. A ball screw device, comprising: a ball screw shaft; a ball screw nutinto which the ball screw shaft is screwed, the ball screw nut beingformed with a plurality of raceways on an outer peripheral face thereof;a first cylindrical member disposed in a periphery of the ball screwshaft, and having an end portion spline-fitted with one end portion ofthe ball screw nut so as not to rotate with each other; balls arrangedso as to form a plurality of rows and rotatable on the raceways of theball screw nut; and a second cylindrical member rotatably supporting theball screw nut and the first cylindrical member by way of the balls. 2.The ball screw device as set forth in claim 1, wherein: the firstcylindrical member is formed with a rotor of a motor; and the secondcylindrical member is formed with a stator of the motor.